... sed primo dubitavit et negavit se de his rebus responsurum esse. I don't have any trouble translating this sentence but rather I question the Latin wording. Shouldn't the next to last word be responsurus(masculine)?

It's masculine, but accusative, agreeing with its subject, se. It's part of the acc + inf. construction which I'm sure you've come across before and which regularly follows a verb of speech or thought or perception.